Communication devices such as User Equipments (UEs) are also known as e.g. terminals, mobile terminals, wireless terminals and/or mobile stations. User equipments are enabled to communicate wirelessly in a cellular communications network or wireless communication system, sometimes also referred to as a cellular radio system or cellular networks. The communication may be performed e.g. between two user equipments, between a user equipment and a regular telephone and/or between a user equipment and a server via a Radio Access Network (RAN) and possibly one or more core networks, comprised within the cellular communications network.
User equipments may further be referred to as mobile telephones, cellular telephones, laptops, or surf plates with wireless capability, just to mention some further examples. The user equipments in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the RAN, with another entity, such as another terminal or a server.
The cellular communications network covers a geographical area which is divided into cell areas, wherein each cell area being served by a base station, e.g. a Radio Base Station (RBS), which sometimes may be referred to as e.g. “eNB”, “eNodeB”, “NodeB”, “B node”, or BTS (Base Transceiver Station), depending on the technology and terminology used. The base stations may be of different classes such as e.g. macro eNodeB, home eNodeB or pico base station, based on transmission power and thereby also cell size. A cell is the geographical area where radio coverage is provided by the base station at a base station site. When a user equipments moves, it measures on signals transmitted by one or more base stations for specific cells. The user equipment can thereby determine the signal strength for a particular radio cell. The measurement results are typically reported to the base station. When the quality of a signal of another radio cell than the one that the user equipment is currently connected to, exceeds by more than a threshold, compared to the signal quality of the cell that the user equipment is currently connected to, a handover to the stronger cell is typically performed for the user equipment. A handover procedure typically comprises measurements performed by the user equipment, measurement reports transmitted by the user equipment to a network node, e.g. a base station, a handover logic determining the target cell for the user equipment, handover preparation signalling towards the network node controlling the target cell, handover signalling towards the user equipment to instruct it to execute handover, possibly comprising also parameters that have been obtained in the handover preparation procedure. Different variants of handover are known in the art. One base station, situated on the base station site, may serve one or several cells. Further, each base station may support one or several communication technologies. The base stations communicate over the air interface operating on radio frequencies with the user equipments within range of the base stations. In the context of this disclosure, the expression Downlink (DL) is used for the transmission path from the base station to the user equipment. The expression Uplink (UL) is used for the transmission path in the opposite direction i.e. from the user equipment to the base station.
In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), base stations, which may be referred to as eNodeBs or even eNBs, may be directly connected to one or more core networks.
Universal Mobile Telecommunications System (UMTS) is a third generation mobile communication system, which evolved from the GSM, and is intended to provide improved mobile communication services based on Wideband Code Division Multiple Access (WCDMA) access technology. UMTS Terrestrial Radio Access Network (UTRAN) is essentially a radio access network using wideband code division multiple access for user equipments. The 3GPP has undertaken to evolve further the UTRAN and GSM based radio access network technologies.
3GPP LTE radio access standard has been written in order to support high bitrates and low latency both for uplink and downlink traffic. All data transmission is in LTE controlled by the radio base station.
Sometimes, many user equipments are traveling together as a group. This may be the case when many user equipments are traveling in the same moving object, such as a train, car, boat, etc. When many user equipments move as a group from cell to cell, they simultaneously perform handovers between the cells on their way. Since a handover from one cell to another requires a lot of signalling, this leads to high signaling peaks at cell borders that may lead to dropped calls and connections, reduced data rates, etc.
A solution to the above problem is the introduction of mobile relay nodes comprising mobile cells, to be sited in the moving object, as being addressed in 3GPP TR 36.836. However, a mobile relay has many challenges by itself from the complexity of managing moving relay cells within the 3GPP architecture.